A basic overview of what I've found:
Simply put, there seems to be a vast number of people that have been murdered in Arkham, nowhere else in Massachusetts, just the city of Arkham. Although the police have constantly said that they are unrelated in any way, I have found a few things that link them all to each other.
There are many patterns I've been able to find. Some
of them big, others small, some obvious, others secretive. And some of them made me wonder "How is this even something people missed?". I may be going insane I suppose, but at least I'm doing something worthwhile, at least to me
There's something I've immediately noticed about these various murders: Every time there is a killing spree it happens 12 years after the previous one. The last murder at the time of writing this was in 1992 with the events of the "postal killer", but the one before that was the 1980 "murders of Tanya House" case.
Another thing I've seen is that the police force is extremely secretive about the murders. For instance, they never truly investigated them like the postal killer and never leaked that much info. For example, the most we know about the postal killer is that his first name was "Kenshin". No last name. just Kenshin. Similarly, the 1980 murders of Tanya House were only revealed to be a priest who was the murderer in 1997.
The range of all the victims seems to be between 14 - 20 people. I know I keep bringing up the postal killer but the postal killer was estimated to have murdered around 17 people. One of which was himself after "mutilating his face with a razor" according to authorities.
A final thing I've been able to find about the murders is that all the murderers in one way, shape or form have documented the events that unfolded. again, a famous example is the postal killer, although the actual documents haven't been released yet.